1. Field of the Invention
The present invention relates to an isocyanate-modified photosensitive polyimide. The photosensitive polyimide of the invention possesses excellent heat resistance, chemistry resistance, and flexibility, and can be used as a liquid photo resist or dry film resist, or used in a solder resist, coverlay film, or printed wiring board.
2. Description of the Prior Art
These days, electronic products are made light and small. Therefore, various electronic parts have been miniaturized. Since flexible printed wiring boards are excellent in flexibility and weight, the demand for them has been increased.
With respect to the materials for coverlay film, they can be classified into three types: photosensitive coverlay, non-photosensitive coverlay, and thermal plastic coverlay. Photosensitive coverlay can be polyimide based (PI based coverlays) or non-polyimide based (non-PI based coverlays), while utilization of the non-polyimide based coverlay is limited because it has lower heat resistance and a higher coefficient of thermal expansion. As to non-photosensitive coverlay, its manufacture is more complicated than that of photosensitive coverlay, and it is not as useful as photosensitive coverlay. As to thermal plastic coverlay, it needs post processing for forming holes and is not as convenient to use as photosensitive coverlay.
U.S. Pat. No. 6,605,353 discloses an epoxy-modified polyimide. However, since these polymers are produced by reacting an epoxy and acid, the reactivity is not good and after ring opening, the OH functional groups will react again with the epoxy radicals. Given the above, the epoxy-modified polyimide would have problems associated with stability and heat resistance.
To improve the heat resistance and processing convenience of coverlays, there have been demands in the art for photosensitive polyimides which are free of the above-mentioned drawbacks.